1. Technical Field
The disclosure pertains to a plasma reactor for processing a workpiece such as a semiconductor wafer using RF power of different phases applied to different RF power applicators.
2. Background Discussion
Plasma processing of a workpiece in the fabrication of integrated circuits, plasma displays, solar panels or the like requires uniform or controlled treatment of each workpiece across its surface. For example, in plasma processing of semiconductor wafers, feature sizes are on the order of nanometers, and uniformity and control of plasma ion distribution density across the workpiece surface is critical. Uniformity of distribution of etch rate or deposition rate across the surface of workpiece is required, as workpiece size (e.g., semiconductor wafer diameter) is increasing, and feature sizes are decreasing. Non-uniformity in plasma processing can arise from non-uniformities or asymmetries in the reactor chamber electrical characteristics, non-uniformity in the distribution of process gases and flow rates, or non-uniformity in the application of RF power, for example. It is necessary to correct or compensate for such non-uniformities.
Plasma processing of a workpiece can be controlled by controlling the phase difference between RF power waveforms applied to different RF power applicators of a plasma reactor. The RF power applicators may be, for example, an electrode in the wafer support pedestal and a ceiling electrode overlying the pedestal. The user may select a desired phase difference for process control purposes. RF sensors at the two electrodes are used to measure the actual phase difference, and this measurement is compared to the desired phase difference to produce a phase correction command. A phase controller controlling the RF power source responds to the phase correction command to effect the desired correction.
A problem is that the phase controller is capable of effecting the desired correction only at a limited speed and with limited accuracy due to instabilities. This is particularly true where analog techniques are employed in implementing the phase controller. There is a need for a phase controller that effects the desired correction at a faster speed and that is free of instabilities.